This invention relates generally to scanning tunneling miscroscopy and, more particularly, to an apparatus and method for integrating a scanning tunneling microscope and a time-of-flight spectrometer.
Three forms of microscopy in current use are capable of forming interpretable atomic resolution images. These are known as field-ion microscopy, transmission electron microscopy, and scanning probe microscopy. Two of these prior art methods allow the type (i.e. the atomic number) of the atoms in the image to be determined. In electron microscopy, the transmitted energy loss spectrum may be used to identify clusters of atoms by type in favorable cases. However, it is not possible to determine whether these lie within the sample or on its surface, and the method is restricted to a few types of atoms. Using field-ion microscopy and time-of-flight microscopy (the atom probe), it is possible to identify individual atoms at identifiable sites on a surface. However, the sample must be prepared in the form of an atomically sharp needle, and the range of materials which can be studied is limited.
The ability to identify "foreign" atoms on the surface of materials has been sought in many areas of science, from catalysis and solid state chemistry to the semiconductor industry. In surface science, this capability is desired in order to contribute to the basic understanding of mechanisms in crystal growth and the role of impurities in controlling interface properties.
It is therefore the principal object of the present invention to provide an apparatus and process for identifying atoms at particular predetermined sites on a material surface.
This and other objects are accomplished in accordance with the illustrated preferred emobodiment of the present invention by first identifying the atoms of interest on a sample surface in images formed by a scanning tunneling microscope. Scanning tunneling microscopes are well known in the prior art, exemplary of which is G. Binning and H. Rohrer, "Scanning Tunneling Mircroscopy-from Birth to Adolescence," Reviews of Modern Physics, Vol. 59, No. 3, Part I, July 1987 (pp. 615-625). These atoms are then transferred to the tip of the scanning tunneling microscope. The sample is then removed, and the atoms ejected from the tip into a conventional time-of-flight spectrometer. By measuring the time of flight of the atoms from the tip to a channel-plate detector, the atomic number of the atoms may be determined.
Unlike the prior art atom probe described by E. Muller et al., "The Atom-Probe Field Ion Microscope," The Review of Scientific Instruments, Vol. 39, No. 1, January 1968 (pp. 83-86), the apparatus and method of the present invention allows large flat samples to be used, and the range of materials which can be studied is almost unlimited. Unlike microanalysis in scanning transmission electron microscopy, the atoms are known to lie on one surface, and there is no limitation on the types of atoms which can be identified.